自动把纯文本转换成Web页面的php代码

首先让我们来看一个我朋友希望转换的纯文本文件的例子：
以下为引用的内容：

代码如下:

　　Green for Mars!
　　John R. Doe
　　The idea of little green men from Mars, long a staple of science fiction, may soon turn out to be less fantasy and more fact.
　　Recent samples sent by the latest Mars exploration team indicate a high presence of chlorophyll in the atmosphere. Chlorophyll, you will recall, is what makes plants green. It's quite likely, therefore, that organisms on Mars will have, through continued exposure to the green stuff, developed a greenish tinge on their outer exoskeleton.
　　An interview with Dr. Rushel Bunter, the head of ASDA's Mars Colonization Project blah blah...
　　What does this mean for you? Well, it means blah blahblah...
　　Track follow-ups to this story online at http://www.mars-connect.dom/. To see pictures of the latest samples, log on to http://www.asdamcp.dom/galleries/220/

相当标准的文本：它有一个标题、一个署名和很多段的文字。把这篇文档转换成为HTML真正需要做的是使用HTML的分行和分段标记把原文的布局保留在Web页面上。特殊的标点符号需要被转换成为对应的HTML符号，超链接需要变得可以点击。
下面的PHP代码(列表A)就会完成上面所有的任务：
列表A
让我们来看看它是如何工作的：

代码如下:

// set source file name and path
$source = "toi200686.txt";
// read raw text as array
$raw = file($source) or die("Cannot read file");
// retrieve first and second lines (title and author)
$slug = array_shift($raw);
$byline = array_shift($raw);
// join remaining data into string
$data = join('', $raw);
// replace special characters with HTML entities
// replace line breaks with

$html = nl2br(htmlspecialchars($data));
// replace multiple spaces with single spaces
$html = preg_replace('/ss+/', ' ', $html);
// replace URLs with elements
$html = preg_replace('/s(w+://)(S+)/', ' ', $html);
// start building output page
// add page header
$output =<<
HEADER;
// add page content
$output .= "

$slug

";
$output .= "

By $byline

";
$output .= "

$html

";
// add page footer
$output .=<<